Journal of the Geological Sociefy, London, Vol. 148, 1991, pp. 1091-1100, 9 figs Printed in Northern Ireland

Progressive deformation of the South Stack and New Harbour Groups, Holy Island, western , North Wales

EMRYS PHILLIPS Department of Geology, University of Wales, PO Box 914, Cardif Present address: British Geological Survey, West Mains Road, Edinburgh EH93LA, UK.

Abstrad: The MonianSupergroup of westernAnglesey comprises three major lithostratigraphic units: the South Stack (oldest), New Harbour and Gwna Groups. In northern and western Anglesey, deformationand metamorphism each increase in intensity downward through the Monian Super- group, especially towards the base of the New Harbour Group. The variation in style and apparent intensity of deformation recorded by the South Stack and New Harbour Groups were governed by gross lithological differences. A correlation is proposed between the deformation histories of the New Harbourand South Stack Groups. Early deformation of thesupergroup produced a pervasive bedding-parallel S1 fabric (chlorite) in both the New Harbour and, to a lesser extent, South Stack Groups.Development of the major, SE-verging F2 PenrhynMawr and anticlines, and associated minor folds also produced an axial planar S2 pressure solution and crenulation cleavage (phengite).The Rhoscolynanticline was subsequently modified by thelocalized development of recumbent, mesoscale F3 folds with an axial planar pressure-solution cleavage (phengite to phengitic muscovite). The F3 event may have accompanied an episode of thrust faulting. Deformation of the South Stack and New Harbour Groups was accompanied by a progressive regional greenschist facies metamorphic event. Thesimplest interpretation of the deformation history of the Monian Supergroup involvesaprogressive SE-directed shearevent whichoccurred prior to the deposition of an Ordovician overstep sequence on Anglesey.

Rocksbelonging tothe MonianSupergroup (Shackleton Rhoscolyn anticline. Although Cosgrove(1980) recognized 1975) (LatePrecambrian to earlyCambrian) crop out the similarities between the deformation histories of the extensively in western Anglesey, where they are unconfor- South Stack and New Harbour Groups, the exact nature of mablyoverlain by an Ordovician(Fennian) overstep the correlation and the effect of his early D1 event upon the sequence (Beckly 1987). The Monian Supergroup comprises New Harbourmetasediments remained unclear. Recently, three major lithostratigraphic units. The basal South Stack Lisie (1988) has recognized that bedding/cleavage vergence Group(>loo0 m), sequencea of interbeddedmeta- relationships, in the Rhoscolyn area, have been sandstones,pelites and subordinate (10-25 m modified/reversed during a later phase of folding and as a thick), is further subdivided into three formations: the South result questioned the existence of the early SE-dipping S1 Stack (oldest), and Rhoscolyn Formations (cf. fabric of Cosgrove (1980). Shackleton 1975) (Fig. 1). These are overlain by a 2-3 km The primary objectives of this paper are to elucidate the thick sequence of highly deformed pelites, semipelites and deformation histories of the South Stack and New Harbour subordinatemetabasic rocksknown as the New Harbour Groups,the age of deformationand toerect adetailed Group, which is in turn overlain by the Gwna Melange. correlationbetween the deformation histories of the two Barber & Max (1979) emphasized the differences in both groups. A SE-directed, progressive deformation model is style and intensity of deformationrecorded at different proposed to explain the deformation history of this part of structural/stratigraphic levels within thesupergroup and the Monian Supergroup.Although standard symbolshave argued forthe presence of amajor thrust separating the been used (e.g. S1, S2) it is not intended that successive sets South Stack and New Harbour Groups. They suggested that of structures necessarily imply a separate deformation event the New Harbour Group was deformed prior to its tectonic (Dl, D2. . . Dn). For a more detailed description of the emplacement upon an undeformed South Stack Group. This deformationstructures developed in the South Stack and interpretation was contested byPowell andothers (in New HarbourGroups the reader is referred to Phillips discussion of Barber & Max1979) whointerpreted the (1989). differences in style andapparent intensity of deformation recorded by the South Stack and New Harbour Groups as being governed by lithological contrastsbetween the two groups. Minor structures in the South Stack Group Cosgrove(1980) concentratedon the minorstructures The South Stack metasediments are well exposed in the cliff developedin the South Stack Group associated with the sections along the SW coast of Holy Island (Fig. 1). Several major SE-vergingRhoscolyn anticline (Fig. 1). He generations of cleavage,lineations and foldshave been concluded that a NW directed D1 event produced an early, recognizeddeforming thesemetasediments (Phillips 1989) lowangle SE-dipping fabric, whichwas thendeformed (Fig. 2). In general, the intensity of thesestructures duringlatera SE-directed D2 eventthat formed the increases towardsthe SE of Holy Island. Other minor 1091

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84

83

82

81 Nev Harbour Group

80 gabbro - serpentinite

South Stark Group @ Rhoscolyn Fm (-topleft)

Holyhead Quartzite

Holyhead Fm (quartzite - top left)

South Stark Fm ,F:;: .:

fault 0 2km I I J thrust 25 26 27 26 Fig. 1. Simplified geological mapof Holy Island. Inset: map of Anglesey showing location of study area. B.W.T, Breakwater thrust; P.M.A, Penrhyn Mawr anticline; P.G.T., Porth y Garan thrust; R. A, Rhoscolyn anticline (Phillips 1989).

structuresinclude sigmoidal, en-echelon quartztension theseunits being dominated by S2 fabricdevelopment fissures, cusp and mullion structures, and thrusts. [SH25967620]. In thepelites, F2 resultedin the An early bedding-parallel S1 fabric (chlorite) is primarily deformation of S1 and a set of early quartz-tension fissures developed/preserved within the meta-siltstonesand fine- by recumbent, asymmetrical folds. Thequartz tension- grainedmeta-sandstones. This fabric is defined by fissures indicate aconsistent SE-directedshear sense on alternating (C1.5cm)quartz- and mica-lithons (Fig.3a) bothlimbs of, forexample, the Rhoscolyn anticline and, [SH 2625 75101, with S1 developmentresulting in the therefore, did notdevelop bya process of flexural slip intensification of awell developed sedimentary lamination associated with F2 folding (cf. Cosgrove 1980). (Fig. 3a). A NW-SE-trending L1 minerallineation On thenorthern limb of the Rhoscolyn Anticline (chlorite) is exposed on Sl/SO surfaces. No convincing F1 [SH26007625 to SH 2660 75101, S2 is locally deformed by fold phasehas been identifiedwithin the SouthStack recumbent, SE-verging, NE-plunging (Fig. 2) F3 folds (Fig. Group. 4) and an associated axial planar S3 pressure-solution fabric Beddingand S1 weresubsequently deformed during a (phengiteto phengitic-muscovite). The main effect of this pervasive F2 fold event. F2 folds are gently NE-plunging later fold phase was to modify and locally reverse SO/S2 (Fig. 2), upright to recumbentstructures exhibiting Z, M vergence relationships (cf. Lisle 1988). Deformation of S2 and S geometries relative to their position on intermediate during F3 wasaccomplished internal to beddingwith SO and large scale F2 structures. An associated, pervasive, S2 being relatively undeformed (Fig. 4). The axial surfaces of pressure solution cleavage(with phengitegrowth) may be F3 within the psammites are sub-parallel to earlier F2 axial planar to minor F2 folds, or weakly transecting as well structuresin the adjacentpelites, with F3 resulting in a asforming convergent and divergent cleavage fans. In tightening of these earlier structures and the superimposi- lithologies in which S1 is well developed, S2 is characterized tion of S3 upon awell developed S2 transposition fabric. by both a pressure solution and crenulation cleavage (Fig. Lisle (1988) emphasized the importance of identifying 3a). Meso-scalefolding of the massive orthoquartzites is modified to reversed bedding/clevage vergence relationships limited [SH 2064 8225; SH 2600 76251, with deformation of in foldedterranes. The locally SE-dipping fabric, which

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N N N

South Stack Group N N N

New HarbourGroup ...'.. . '. .: :: .:( . . .. , .:. 15+ contour 1-4 H 5-9 .. 10-14 Fig. 2. Representative structural data recorded from the South Stack and New Harbour groups. Equal-area projections.

Cosgrove(1980) interpretedas having beendeveloped bedding-parallel fabric (chlorite), and a second NW-dipping during his early NW-directed D1 event, is now interpreted S2 crenulation cleavage (Fig. 2). Bedding within the group as being due to thereversal of SOIS2 vergence during F3. has largely been transposed into the S1 fabric [SH 2510 7900 In the core of the Rhoscolyn Anticline, asymmetrical to to SH242079551. The mineralogy,morphology and symmetrical minor F2 folds were modified during F3 (Fig. metamorphicgrade of S1in the New HarbourGroup is 3b) [SH 250 7501. Hinge migration occumng in response to comparable tothat of the earlybedding-parallel fabric this later fold phase resulted in the localized folding of S2 identified in the underlying South Stack Group. and the reorientation of this fabric on the overturned limbs Associatedwith S1 is well-developeda L1mineral into parallelismwith the developing S3 fabric.Conse- (chlorite) lineation and quartz elongation lineation (Fig. 2 quently,S3 was superimposedupon S2, leading tothe and Fig. 3c). Quartz tension-fissures are commonand observed increase in intensity of the fabricon the exhibita SE-directedshear sense comparable to similar overturned limbs of these fold structures (Fig. 3b). structures in the South Stack Group. Despite the intensityof Also developed in the South Stack Group, affecting to S1 in the New HarbourGroup, no F1 folds havebeen varying degrees all lithologies, isa set of conjugate kink recognized. bands which are broadly coaxial to F2 and F3. The intensity F2 folds dominate within the New HarbourGroup, of thesestructures varies considerably,and they clearly deforming the earlier S1 and L1 lineation (Fig. 3c and d), post-date all the earlier folds and fabrics. withL1 lying approximatelyorthogonal to later F2 fold hinges (Fig. 3c). F2 folds are predominantly NE-plunging, SE-verging (Fig. 2),recumbent non-cylindrical structures Minor structures in the New Harbour Group with an axial planar S2 crenulation cleavage. On the basis of Overlying the South Stack Group are the highly deformed, fold interferencethree generations of F2 folds havebeen dominantly pelitic chlorite-mica-schists of the New Harbour identified. All thesegenerations deform S1 and exhibit a Group (Fig. 1).Two generations of fabricshave been consistent SE-directedshear sense with an axial planar identified deformingthese metasediments; and early crenulation cleavage (phengite) of equivalent metamorphic

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b

d

e 'E W

Fig. 3. (a) Well-developed bedding-parallelS1 fabric within a metasiltstone deformedby a spaced S2 crenulation cleavage(pencil is 15 cm long) (South Stack Group).(b) SE-verging F2 fold modified duringF3 resulting in the localized folding of S2 in the hinge of the fold (South Stack Group). (c) F2 fold deformingS1 and L1 in the New Harbour Group. (a) SE-verging F2 fold developed within theNew Harbour Group. (e) Bedding-paralllel thrust developed along the upper boundaryof a quartzite unit, South Stack Group[Sh 2600 76551 (cliff is 10-15 m high). (f) Post-F2 thrust fault deforming the South Stack Group [SH2365 79551.

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Fig. -(continued)

NW S.E, grade.Consequently, it is concluded that all three fold generations developed during the same progressive defor- mation event. Finally, a late set of conjugate kink bands deform both S1 and S2 cleavages. The intensity of kink band development varies considerablyand is comparable to structures of asimilar agedeveloped in theSouth Stack Group.

Major fold structures Two major, SE-vergingkilometre-scale anticlines deform boththe South Stack and New Harbour Groups (Fig. 1). They are separated by a broad syncline occupied by the New Harbour Group. These are, in the north the Penrhyn Mawr anticline and to the south the Rhoscolyn anticline (Figs 1, 5 Fig. 4. Field sketch of an F3 fold showing the modificationto and 6). The essential features of the NE-plunging Penrhyn localized reversal ofSO/S2 vergence relationships.0 = angle Mawr anticline are illustrated in Fig. 5. The relative position between bedding andS3/F3 axial surface (scalebar is 25 cm). and attitude of both the major anticline and intermediate

PENRHYN MAWR AECLINE N. W -- -- S. E NEW HARBWR GROUP -?&&d Mountain '- .-

r

Fig. 5. Simplified cross section through the Penrhyn Mawr anticlinefrom South Stack [SH 2048 82301 to [SH 2335 80001.

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RHOSCOLYN ANTICLINE S.E NEW HARBOUR GROUP

l ; < =way UP / / =S2 0 MOm td ’.’=S3

4 Fig. 6. Simplified cross section through PORTH SAINT RHOSCOLYN HEAD COAS TGURO PORTH Y COWGl the Rhoscolyn anticline.

scale structureswere determined by the variation in consistentwith the Rhoscolyn anticline being an F2 measured bedding attitude, F2 fold vergence and S, M and structure. The intermediate relationship of the dip of the Z fold-geometries, SO/S2 vergencerelationships and axial plane of the anticline to the mean dip of S2 and S3 is younging directions determined for well preserved sedimen- considered to be dueto the partialrotation of its axial tary.structures. The calculatedorientation of thismajor surface toward the SE during F3. Rotation of this major anticline is comparableto the meanplunge and axial anticline toward the SE also resulted in the modification of orientation of bothminor F2 folds and L2 lineation minor F2 folds within its core. developedin the SouthStack Group; consequently the The lack of exposure of the South Stack Group between Penrhyn Mawr Anticline is of F2 age. Bwa Du [SH 2600 76251 and Porth Dafarch [SH 2335 80001, At Rhoscolyn,however, initial SOIS2 vergenceand in conjunction with the highly deformed nature of the New minor F2 fold relationshipshave been modifiedby F3 HarbourGroup, makes the interpretation of the gross resulting in a more complex structural history. Whilst the structure of this area problematical.Indirect evidence for geometry of the minor F2 folds relative to their position on the structure of this area is afforded by the resistivity study the anticline suggests that the Rhoscolyn anticline is F2 in of Habberjam & Jackson (1974). The increase in resistivity age, SOIS2 vergencerelationships are variable on asmall with depth and the symmetry about Bay of high scale, especially on the northern limb of the anticline due to resistivity values, indicative of the psammitic South Stack F3 folding. If, however, SO/S2 relationships are ignored in Group (Fig. 7) (for details see Habberjam & Jackson 1974), intervalswhere F3 folds are conspicuous,then S2 is suggests that the Penrhyn Mawr and Rhoscolyn anticlines

N.W S. E Holyhead Mt Trearddw Bay Cymyran by

a soundmo locahons

0 1- 2 3 4km

< 400 c 630 I7.J I7.J c 1000 S.:. ” < 7580 0 ...... ,.L.

ENRHYNMWR ANTICLINE RHOSCOLYNANTICLINE New HarbourGroup b c] RhOsC-OIYn Fm. Fig. 7. (a) Contouredplot of resistivity along a line from to Ho/@ad Fm. CymranBay (HolyIsland) (Habberjam & Jackson 1974). (b) Simplified cross 3 1 m South Stack Fm. sectionthrough Holy Island based on Lu 0 1 2 3 4km the resistivity dataof Habberjam & P Jackson (1974) and field data.

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arelinked by amajor syncline occupied by the lower resistivity New Harbour Group. bntfle hrnh bands thrusts Thrust faulting Two episodes of thrusting have been recognized: an early phase of bedding-parallel thrusting, and a late post-F2 phase of brittle thrust faulting (Phillips 1989). Greenly (1919) was the first to recognize the presence of bedding-parallel thrusts along the upper boundaries of the quartzites (Fig. 3e)within the South Stack Group (e.g. the Breakwater thrust. Fig. l), although subsequent workers (e.g. Cosgrove 1980) have not discussed these structures. These early thrusts are deformed by later F2 folds [Sh2600 76251 placing a relative age on initial thrusting as pre-F2.However, the increase in the intensity of S2 towards the top of the quartzites suggests that Fig. 8. Correlation between the deformation historiesof the South movement continuedalong these structures during F2, Stack and New Harbour Groups. N, data collected from northof possibly in response to layer-parallel flexural slip. Trearddur Bay; S, data collected fromsouth of Trearddur Bay. A second phase of post-F2 brittle thrusts deforms both the South Stack and New Harbour Groups, and includes the 2m of thecontact. A pervasivefabric developed within Trearddur Bay [SH225 7881 andPorth y Garanthrusts these rocks occurs parallel to both the contact and S1 in the [SH 256877251 (Fig. 1) (after Greenly 1919). The sense of New Harbour Group. This foliation, along with the contact, overthrusting on these structures is towards the SE (Phillips was then deformed during the F2 fold phase [SH 2400 80351. 1989). In the South Stack Group the intensity and number Kinematicindicators within the graphitic-schists of these NW-dipping thrustsincreases between Porth [SH 240080351 (e.g.rootless folds, tension-fissures and Dafarch [SH2335 80001 and Porth y Corgwi [SH2202358] shear bands) yield a SE-directed shear sense prior to F2. (Fig. l). The intensity of thrusting in this area is atypical of The intensity of S1 increases downward through the New the South Stack Group, with both the meta-sandstones and Harbour Group, especially within the pelites and metabas- pelites becoming highly schistose in the vicinity of these ites at the base of the group. Consequently, the boundary structures (Fig. 30. The Trearddur Bay Thrust is considered between theSouth Stack and New HarbourGroups to have deformed thecontact between the New Harbour representsahigh-strain zone. The grossdifferences in and SouthStack Groups, based on the presence of fault lithology between the New HarbourGroup and, more bounded lenses of highly deformed graphitic-mica-schist in competent,South Stack Group would haveresulted in the Trearddur Bay area [SH 2521 7901; SH 2535 78731 which higher strains being recordedalong this lithostratigraphic are commonly observedmarking the contact between the boundary. two groups [SH 2400 80351 (Phillips 1989). It is possible that thrusting in the South Stack Group (between Porth Dafarch and Porth y Corgwi) represents the Trearddur Bay Thrust at Progressive deformation model a deeper level within the Monian Supergroup. Previous workers,including Barber & Max (1979) and Cosgrove (1980), have interpreted the deformation histories of the South Stack and New Harbour Groups in terns of Correlation between the deformation of the South several discretedeformation events. A detailed study of Stack and New Harbour Groups thesestructures (Phillips 1989), however,has led toa A direct correlation can be made between the deformation reappraisal of thedeformation history of the Monian histories of the South Stack and New Harbour Groups (Fig. Supergroup exposed inSW Anglesey. Successive gener- 8), supported by the broadlycoaxial nature and the ations of ductile structures developedin the South Stackand consistentSE-directed shear sense obtained for successive New Harbour Groups are broadly coaxial (Figs 2 and 8). generations of minor structures developed in both groups. For example, L1 in the New HarbourGroup lies Barber & Max (1979) argued for the presence of a major approximately orthogonal to later F2 fold hinges (Fig. 3c) thrustseparating the South Stack and New Harbour i.e. parallel to the stretching direction of this fold phase. Groups, with the New HarbourGroup having been Furthermore,consistenta SE-directed shear sense is deformedprior to its tectonicemplacement upon an obtainedfor these various ductile structures, whichis undeformedSouth Stack Group.However, the proposed mimicked by the SE sense of overthrusting established on correlation invalidates this conclusion. The identification of the later brittle thrusts. a previously unrecognized bedding-parallel S1 fabric in the Consequently,the simplest interpretation of the South Stack Group provides important new evidence that deformation history of the South Stack and New Harbour thesegroups have undergone identical deformation Groups isin terms of one progressiveSE-directed shear histories. event. This event can be best illustrated in terms of three The boundary between the two groups is marked by a progressive stages. distinct change in both lithology and apparent intensity of Stage 1 (Fig. 9). The initial stages of deformation were deformation.Associated with thiscontact are a series of dominated by shearingconcentrated within the New discontinuous,highly-sheared metabasites and graphitic HarbourGroup and pelitic horizons of the SouthStack schists (Fig. 1) [SH 27007464; SH 2400 80351, which either Group, Shearingresulted in thedevelopment of a occur at this boundary or in the New Harbour Group within bedding-parallel S1 fabric in both groups, accompanied by

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STAGE l NW S.E directed shear S.E

STAGE 2 New HarbourGroup

South Stack Group Rhoscolyn Fm.

Holyhead Fm.

S.E 2km STAGE 3 nrhyn Mawr Anticline Rhoscolyn Anfiche vertical = horizontal scale

Slmp//fied Crosssection [effectsof later fauitmgremoved l

Fig. 9. Cartoon illustrating the progressive deformation history proposed for Newthe Harbour and South Stack Groups(a detailed cross section through the SW coast of Holy Island is available as Supplementary PublicationNo SUP 18071 (one A3 page) from the British Library, Document Supply Centre, Boston Spa, Wetherby, West YorkshireLS23 7BQ and from the Society Library. Itis also availableon request from the author).

thelate development of sigmoidal quartz-tension fissures. arisenfrom this study regards the validity of the use of The competence contrasts between the orthoquartzites and standardstructural symbolsprogressivelyin deformed theinterbedded meta-sandstones and pelites led to terranes. Suchsymbols were originally designed to detachment (thrusting) along the upper boundaries of these distinguish between successive generations of minor quartzite units. Gross differences in mechanical anisotropy structuresrelated to severaldiscrete deformation events. betweenthe predominantly psammitic South Stack Group However, in areas where a progressive deformation history and pelitic New Harbour Groupresulted in the development can be established the use of these symbolsmay lead to of a high-strain zone along the boundary between the two confusion. Consequently, with the increasing application of groups. progressive deformation models to complex, deformed Stage 2 (Fig. 9). This stage is characterized by F2 folding terranes (e.g. Key et al. 1991) perhaps a review of this type together with the development of a NW-dipping foliation. of nomenclature isnow required.Unfortunately, such a The Rhoscolyn andPenrhyn Mawr anticlines form,and revision is beyond the scope of this paper. flexural slip associated with the development of these major structures probably resulted in continued movement along earlier thrusts and the New Harbour/South Stack contact. Discussion Stage 3 (Fig. 9). The ductile deformation of stages 1 and Although adirect correlation can bemade between the 2 was superseded by anepisode of SE-directedbrittle deformation histories of the South Stack and New Harbour thrusting. SE-overthrusting of the Penrhyn Mawr anticline Groups, and a similar deformation history recognised in the caused thereorientation of earlier folds andfabric in the New Harbourmetasediments exposed in NWAnglesey hangingwall of this structure.Tectonic thickening of the (Khonstamm 1980), a number of key questions need to be MonianSupergroup led to F3 folding andfabric consideredwith respect tothe tectonic evolution of the development within the South Stack Group and the rotation Monian Supergroup. of the Rhoscolyn anticline toward the southeast. The age of deformation of theMonian Supergroup Thisdeformation event was accompanied by a remains uncertain.An Ordovician (Fennian)overstep progressive,regional greenschist facies metamorphic event sequence on Anglesey provides the only age constraint on (Phillips 1989). Theseearly, dominantly ductile structures deformation.Foliated clasts Newof HarbourGroup werelater deformed by at least one phase of high-angle material in the Ordovician sediments prove at least the early brittle faults, including the , Porth Namarch and fabric within the Supergroup is pre-Fennian in age. Bwa Du faults (Fig. 1). One importantpoint whichhas High-angle brittlefaults which deformearlier-developed

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ductile structures within the supergroup (e.g. North Stack ments may haveoccurred in response to sinistral Fault) can be shown to havecontrolled sedimentation transcurrentmovements along this shearzone during the during the Ordovician(Bates 1972,1974). TheGwna latePrecambrian to earlyCambrian. So although the Melange exposed on the Llyn Peninsula is involved in shear presence of a volcanic arc complex can be established in the zonesthat are overlain by sediments no youngerthan Anglesey area, it remains a possibility that deformation of Moridunian(lower Arenig) age(Beckly 1987). Conse- theMonian Supergroup occurred in response tomajor quently,deformation of the MonianSupergroup occurred transcurrent fault movementsand terrane accretionalong beforeLower Arenig times, but in the absence of the NWmargin of an Avalonian superterrane during late radiometric datathe exactage of deformationremains Precambrian to early Cambrian times. uncertain. Clasts within the Gwna Melange are commonly aligned Conclusions parallel toan early S1 fabric.Gibbons & Ball(1991) described a similar bedding-parallel fabric within the New A directcorrelation can be established between the Harbour Group beneath the Gwna Melange/New Harbour deformation histories of the New Harbour and South Stack contactin N Anglesey. Disruption of the New Harbour Groups,a conclusion supported by thebroadly coaxial Group was interpreted by these authors as having occurred nature and the consistent SE-directed shear sense obtained during the deposition of the Gwna Melange, whilst the New for successive generations of minor structures developed in Harbourmetasediments were still only partially lithified. both groups. Early deformation of the Monian Supergroup This interpretation has important implications for the timing produced a pervasive bedding-parallel S1 fabric in the New of deposition of theGwna Melange with respect to the Harbourand, to alesser extent, South stack Groups. timing of deformation of the underlying New Harbour and Development of themajor SE-verging F2 Penrhyn Mawr South Stack Groups. The comparable, relative age of fabric and Rhoscolyn anticlines,and associatedminor folds also development and postulated bedding-parallel nature of S1 in produced an axial planar S2 pressure solution to crenulation the GwnaMelange with respect tothe deformation cleavage. The Rhoscolyn anticline was subsequently sequence of the Monian Supergroup suggests that melange modified by the localized development of recumbent deposition occurred during the early stages of deformation mesoscale F3 folds with an axial planar S3 foliation. The F3 of the South Stack and New Harbour Groups. event was restricted to the structurally deeper South Stack The Monian metasediments and their associated igneous Group and may have accompanied an episodeof the post-F2 rocks have previously beeninterpreted as recording late thrust faulting. The simplest interpretation of the deforma- Precambriansubduction (see Wood 1974; Barber & Max tionhistory of this part of the MonianSupergroup is in 1979),with theMonian blueschists exposed in eastern terms of one progressive SE directed shear event. The initial Anglesey providing a key piece of evidence for the existence stages of deformation of the Monian Supergroup may have of this subduction system (Thorpe et al. 1984). A accompanied deposition of theGwna Melange. Deforma- collision-accretion process may haveresulted in the tion and melange generation may have occurred in response generation of theGwna Melange and deformation at a to (a) the collision-accretion of a Monian arc system with a deeper level within the supergroup, perhaps similar to the continentalmargin; or (b)during major transcurrent development of the Tiawanese Lichi melange whichwas faulting and terrane accretion along the NW margin of an generated by the collision of anarc systemwith a Avalonian superterrane during the late Precambrian to early continental margin (Page & Suppe 1981). Cambrian. The identification of high-angle ductile shear zones (e.g., the BerwShear Zone in eastern Anglesey) separating This research was carried out during a NERC funded PhD research radically different rock units, led directly to the application studentshipin the department of Geology atthe University of of suspect terranetheory to the Anglesey area (Gibbons Wales, Cardiff. 1983, 1989). In central Anglesey the Monian Supergroup is separatedfrom a belt of gneisses and c. 600 Magranitic References rocks (theCoedana Complex)by majorbrittle faults and BARBER,A. J. & MAX,M. D. 1979. A new look at the Mona Complex ductile shear zones. It is possible that the deformation of the (Anglesey, North Wales). Journal of theGeological Society, London, Monian Supergroup and melange generation may have been l36, 407-432. BATES,D. E. B. 1972. The stratigraphy of the Ordovician rocks of Anglesey. related tomajor transcurrent faulting resulting in the Geological Journal, 8, 29-58. dissection of the Monian arc system, rather than active plate BATES,D. E. B. 1974. The structure of the Lower Palaeozoic rocks of subduction. The correlation of the South Stack Group with Anglesey, with special reference to faulting. Geological Journal, 9, the Cullenstown Formation(Crimes & Dhonau 1967) and 39-60, BECKLY,A. J. 1987. Basin development in North Wales during the Arenig. more recently an enlarged Cahore Group (Tietzsch-Tyler & Geological Journal, 22, 19-30. Phillips 1989) links the MonianSupergroup with thelate COSGROVE,J. W. 1980. The tectonic implications of some small scale Precambrian/LowerPalaeozoic geology of SE Ireland.In structures in the Mona Complex of Holy Island, North Wales. Journal of contrast,no such correlationcan be made between the StruCMral Geology, 2, 383-396. CRIMES,T. P. & DHONAU,N. B. 1967. The Precambrian and Lower MonianSupergroup and the late Precambrian to early Palaeozoic rocks of southeast Co. Wexford, Eire. Geological Magazine, Cambrianrocks of the Welsh mainland. The highly 104,213-221. deformed Cullenstown metasediments occur to the NW of GIBBONS,W. 1983. Stratigraphy, subduction and strike-slip faulting in the the Proterozoic gneisses of the Rosslare Complex (equated Mona Complex of North Wales-a review. 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Received 17 October 1990; revised typescript accepted 10 June 1991.

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